1. Field of the Invention
The invention relates to a time-division digital transmission system of the kind in which a number of component digital channels each having a predetermined flow rate are multiplexed in time to form a single channel adapted to a transmission line.
2. Description of the Prior Art
Systems of this kind are known in the prior art; for instance, the U.S. Pat. No. 3,987,248 filed Nov. 25, 1974 describes a system of the kind which converts 16 incoming component channels transmitting plesiochronic digital signals at the rate of 8.448 Mbits/s into an outgoing channel at 140 Mbits/s and, in the reversed direction, converts an incoming channel at 140 Mbits/s into 16 outgoing channels at 8 Mbits/s. On each incoming channel, the prior-art system comprises an input junctor, that is a circuit which performs a number of functions: i.e. conversion of the code used on the line into the binary code used in the equipment devices, re-shaping of the line signals and restitution of a rate signal. The input junctors are connected by input junctions to store and stuffing circuits, which are connected to a multiplexer. The resulting 140 Mbits/s signal coming from the multiplexer is not suitable for transmission along the line. In the prior art, it is transmitted via an output junction to an output junctor. The output junctor serve the same purpose as the input junctors but operate in the reversed direction, i.e. they effect conversion of the binary code used in the equipment devices into the code used on the line, re-shaping of the signals and restitution of the rate.
In practice, various line codes may be used, such as the bipolar code, the HDBn code and the 4B3T and 4S43 codes. As is known, the bipolar code is a three-level code in which the binary 1 is alternately represented by +1 and -1. The HDBn code is derived from the bipolar code but modified so that no sequence containing more than a number n of zero signal elements can exist. Accordingly, each sequence of (n+1) zeros is represented by a sequence B O . . . OV or O O . . . O V where B is a 1 which respects bipolarity and V is a 1 which violates bipolarity.
In the 4B3T code, each of the 16 four-bit binary words corresponds to one or the other of two ternary words, depending on whether the numerical sum or cumulative disparity of the ternary signals is positive of negative, when measured at the end of the last converted word. The 4B3T code has six ternary words with zero disparity, ten ternary words of positive disparity and then ternary words of negative disparity. Thus, six of the sixteen binary words can be represented by a ternary word of zero disparity and the remaining ten binary words can be represented by ternary words having a positive or negative disparity depending whether the cumulative disparity is itself negative or positive. More specifically, there are two ternary code alphabets, the first alphabet containing all the zero disparity words and all the positive disparity words, whereas the second alphabet, in the same positions, contains the zero disparity words and the negative disparity words which, in the same positions, are the inverses of the positive disparity words. The cumulative disparity can take eight states numbered from 0 to 7, but varies only from 1 to 6 at the end of every ternary word. The first alphabet is used for conversion when the cumulative disparity is equal to 1, 2, or 3 and the second alphabet when the cumulative disparity is equal to 4, 5 or 6.
The following table I shows the two alphabets of the 4B 3T code:
TABLE I ______________________________________ First Second Binary alphabet Disparity alphabet Disparity ______________________________________ 0 0 0 0 0 - + 0 0 - + 0 0 0 0 1 - + 0 0 - + 0 0 0 0 1 0 - 0 + 0 - 0 + 0 0 0 1 1 + - + + 1 - + - - 1 0 1 0 0 0 + + + 2 0 - - - 2 0 1 0 1 0 + 0 + 1 0 - 0 - 1 0 1 1 0 0 0 + + 1 0 0 - - 1 0 1 1 1 - + + + 1 + - - - 1 1 0 0 0 0 + - 0 0 + - 0 1 0 0 1 + - 0 0 + - 0 0 1 0 1 0 + 0 - 0 + 0 - 0 1 0 1 1 + 0 0 + 1 - 0 0 - 1 1 1 0 0 + 0 + + 2 - 0 - - 2 1 1 0 1 + + 0 + 2 - - 0 - 2 1 1 1 0 + + - + 1 - - + - 1 1 1 1 1 + + + + 3 - - - - 3 ______________________________________
In the MS43 code, each of the sixteen four-bit binary words corresponds to one of the three ternary words. Six of the sixteen binary words are always represented by the same six ternary words having zero disparity, and the remaining ten binary words are represented by the same ternary word having non-zero disparity and the same sign in two of the alphabets, or into a ternary word in the third alphabet having a different non-zero disparity but the opposite sign. The cumulative disparity can take six states numbered from 0 to 5, but varies only between 1 and 4 at the end of every ternary word. The first, second or third alphabet is chosen, depending on whether the cumulative disparity is equal to 1, 2, 3 or 4.
The following table II shows the three alphabets in the MS43 code:
TABLE II ______________________________________ First Dis- Second Dis- Third Dis- Binary alphabet parity alphabet parity alphabet parity ______________________________________ 0 0 0 0 + + + + 3 - + - - 1 - + - - 1 0 0 0 1 + + 0 + 2 0 0 - - 1 0 0 - - 1 0 0 1 0 + 0 + + 2 0 - 0 - 1 0 - 0 - 1 0 0 1 1 0 - + 0 0 - + 0 0 - + 0 0 1 0 0 0 + + + 2 - 0 0 - 1 - 0 0 - 1 0 1 0 1 - 0 + 0 - 0 + 0 - 0 + 0 0 1 1 0 - + 0 0 - + 0 0 - + 0 0 0 1 1 1 - + + + 1 - + + + 1 - - + - 1 1 0 0 0 + - + + 1 + - + + 1 - - - - 3 1 0 0 1 0 0 + + 1 0 0 + + 1 - - 0 - 2 1 0 1 0 0 + 0 + 1 0 + 0 + 1 - 0 - - 1 1 0 1 1 0 + - 0 0 + - 0 0 + - 0 1 1 0 0 + 0 0 + 1 + 0 0 + 1 0 - - - 1 1 1 0 1 + 0 - 0 + 0 - 0 + 0 - 0 1 1 1 0 + - 0 0 + - 0 0 + - 0 0 1 1 1 1 + + - + 1 + - - - 1 + - - - 1 ______________________________________
The bipolar code and the HDBn code have the same flow rate as the binary code, but the 4B3T code and its variant, the MS43 code, in which four-bit binary words correspond to words having three ternary digits, result in a decrease in the flow rate in line code compared with the flow rate in the transmission and reception equipment. The flow rate in line code is 3/4 of the flow rate in the equipment. According to the invention, code 4B3T or MS43 is used as the line code and, as will be explained hereinafter, advantage is taken of the reduction in the line flow rate obtained by means of these codes.
It is clear from the preceding summary of the prior art that the transmitting and receiving equipments in multiplex transmission systems are connected to the transmission line by input and output junctors which, among other functions, "transcode" (i.e. convert the code of) information from the binary code into the line code and vice versa. The transcoding, in the case of code 4B3T or MS43, is via parallel four-bit words when transcoding the binary code into the line code, and via parallel ternary three-digit words when transcoding from the line code into the binary code. In the prior-art systems, the bits are sent in "series" from the multiplexer to the transcoder via the output junction, or from the transcoder to the demultiplexer via the input junction. If the HDBn code is used as the line code, transcoding occurs in series; if code 4B3T or code MS43 is used as the line code, the transcoder brings about series-parallel conversion followed by transcoding proper, itself followed by parallel-series conversion.